The invention relates to a chemically amplified photoresist that is particularly suitable for exposure with light having a wavelength of 157 nm, and to a process for structuring substrates, especially silicon wafers.
In order to raise the calculating speed of processors and the capacity of memory elements, and to reduce the costs of the components, the semiconductor industry is developing chips having ever smaller features and hence an ever increasing density of components. One particular challenge is reducing the minimum feature size. In optical lithography, these requirements have been met to date through the transition to ever shorter wavelengths. However, at a feature size of 100 to 70 nm, the existing processes, which use wavelengths as short as 193 nm, approach the limit of their resolution. Consequently, the development of new processes is needed. Particularly good prospects for industrial use are possessed by optical lithography where exposure is carried out using radiation with a wavelength of 157 nm, since in this case the chip manufacturers are able to continue utilizing their extensive knowledge of optical lithography. A key difficulty in the use of an exposure radiation having a wavelength of 157 nm is the insufficient transparency of the presently used materials. For industrial application, the transparency of the base polymer in these high-resolution resists must be as high as possible, while the photochemicals with which, for example, an acid is produced in the resist must have a high quantum yield.
Some of the resists presently used for the manufacture of microchips operate with what is known as chemical amplification. In such resists, exposure in a photoreaction changes the chemical structure of the resist. In the case of a positive-working chemically amplified resist, for example, exposure generates a strong acid that brings about catalytic conversion or cleavage of the resist in a subsequent heating step. As a result of this chemical reaction, the solubility of the polymer in a developer is drastically changed, so that a marked differentiation between exposed and unexposed areas is possible.
The structured (i.e. patterned) photoresists can be used as masks for further operations, such as dry etching operations, for instance. Where the photoresist is used to structure an underlying organic-chemical medium, such as in two-layer resists, the topmost photoresist layer is required to have a high etch resistance. To this end, either the photoresist may contain corresponding etch-resistant groups in the polymer chain, such as silicon-containing groups, or the photoresist is reinforced in terms of its etch resistance in the step following the structuring of the photoresist. For this purpose, the polymer must contain reactive groups as anchor groups. These groups then react with a suitable reactive group of an amplifying reagent, which acts as a linking group, to form a chemical bond. In this way, silicon-containing or aromatic groups can be introduced subsequently into the polymer. The etch resistance of aromatic and organosilicon compounds in an oxygen plasma is much higher than that of aliphatic organic hydrocarbon compounds. Especially for structured resists with a low layer thickness, therefore, subsequent amplification of the resist structures is advantageous. The reaction incorporating organosilicon compounds is often referred to as silylation, the incorporation of aromatic compounds as aromatization.
A process for consolidating structured resists is described, for example, in commonly-owned European Patent EP 0 395 917 B1, which corresponds to U.S. Pat. Nos. 5,234,794 and 5,234,793. In that process, the photoresists used for an exposure wavelength of 248 and 193 nm following their structuring, are chemically reinforced in terms of their etch resistance by incorporating organosilicon groups to form a sufficiently stable etch mask. Where the layer thickness of the resist is sufficient, the structures of the resist can be widened by lateral growth and in that way it is possible to obtain an improvement in the resolution.
As already mentioned, the low transparency of the known photoresists at a wavelength of 157 nm poses a key difficulty in the development of the 157 nm technology. With the existing photoresists, layer thicknesses of approximately 50 nm can be realized. Presently, photoresists are being developed in which fluorination improves the transparency of the polymer at a wavelength of 157 nm. See Patterson et al., Proc. SPIE, 3999 (2000). However, these polymers still have an absorption that is about 50 times higher than that of the polymers commonly used at present in the resists used in industry for exposure with radiation having a wavelength of 193 or 248 nm. Even with these highly fluorinated polymers, layer thicknesses of only about 200 nm are achieved.
Besides high transparency at a wavelength of 157 nm, in order to be useful industrially the resists must meet further requirements, such as, for example, high contrast, good film-forming properties and good developability in conjunction with low basic solubility (dark erosion). Furthermore, the photoresist polymers ought to be very easy to prepare, in order to prevent complex preparation processes that increase the costs of the photoresist.
It is accordingly an object of the invention to provide chemically amplified photoresist and process for structuring substituents using transparency enhancement of resist copolymers for 157 nm photolithography through the use of fluorinated cinnamic acid derivatives that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that provide photoresists having high transparency at a wavelength of 157 nm, high contrast, good film-forming properties and good developability in conjunction with low basic solubility (dark erosion). Furthermore, the photoresist polymers should be very easy to prepare and not require complex preparation processes that increase the costs of the photoresist.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a chemically amplified photoresist including a polymer, a photoacid generator, and a solvent. The polymer contains acid-labile groups that are eliminated under the action of an acid and that liberate polar groups that increase the solubility of the polymer in aqueous alkaline developers. The polymer has first repeating units derived from cinnamic acid or cinnamic esters that are at least monofluorinated and/or substituted by fluoroalkyl groups.
Through the introduction of a first repeating unit derived from an at least partly fluorinated and/or fluoroalkyl-substituted cinnamic acid or cinnamic ester into the polymer, the transparency of the photoresist of the invention is significantly increased; this permits higher layer thicknesses of the photoresist. The aryl units of the cinnamic acid groups considerably raise the etch resistance in the plasma etch operation. Furthermore, with the carboxyl group of the cinnamic acid, a reactive group is introduced at the same time into the polymer. The carboxyl group allows subsequent modification of the photoresist. The incorporation of an at least partly fluorinated cinnamic acid or an at least partly fluorinated cinnamic ester allows a plurality of functions to be introduced into the polymer. In turn, this makes it possible to circumvent the often complicated copolymerization of monofunctional monomers whose copolymerizability with one another is poor. Through the introduction of an at least partly fluorinated and/or fluoroalkyl-substituted cinnamic acid into the photoresist polymer, the following advantages are gained.
a) Fluorination raises the transparency of the photoresist at an exposure wavelength of 157 nm.
b) The introduction of aryl groups into the polymer raises the resistance of the resist toward an etch plasma.
c) The introduction of a carboxyl group, which may also be esterified, provides a group that is able to change its polarity strongly by ester cleavage. Furthermore, the carboxyl group may act as an anchor group, which allows modification of the properties of the polymer and of the photoresist in a consolidation reaction.
The at least partly fluorinated and/or fluoroalkyl-substituted cinnamic acid introduced into the polymer in accordance with the invention has a free-radically polymerizable double bond. It is therefore very easy to integrate into the synthesis of polymers that are already known for use in photoresists, so that adapting known photoresists to lithographic structuring with short-wavelength light can be accomplished with comparative ease.
The other components of the photoresist are already known in general. For instance, as photoacid generators it is possible to use all compounds that liberate acid on irradiation. Use is made advantageously of onium compounds, as described, for example, in commonly-owned European Patent Application No. EP 0 955 562 A1. Preferred photoacid generators are ionic compounds in the form of sulfonium salts and iodonium salts.
Solvents that can be used for the resist include methoxypropyl acetate, cyclopentanone, cyclohexanone, xcex3-butyrolactone, ethyl lactate, diethylene glycol, diethyl ether, ethylene glycol dimethyl ether, dimethyl ether, or a mixture of at least two of these solvents. In general, however, all common solvents or mixtures thereof can be used in which the resist components can be dissolved to form a clear, homogeneous, and storage- stable solution and which ensure good coat quality when the substrate is coated.
In addition to the components stated, the photoresist may further include additional compounds. For instance, the photoresist may additionally include a thermoacid generator. Suitable thermoacid generators include, for instance, benzylthiolanium compounds.
Further components which can be added as additives are those which influence the resist system advantageously in respect of resolution, film-forming properties, storage stability, radiation sensitivity, service life, etc.
The polymer present in the photoresist of the invention contains, pendantly, acid-labile groups that confer on the polymer a low solubility in polar solvents. By the catalytic action of acid and, where appropriate, a simultaneous temperature treatment, polar groups are produced on the polymer. Examples of acid-labile groups that can be used include the following: tert-alkyl ester, tert-butoxycarbonyloxy, tetrahydrofuranyl, tetrahydro-pyranyl, tert-butyl ether, lactone, and acetal groups. Tert-butyl ester groups are particularly preferred.
These acid-labile groups are preferably at least partly fluorinated; i.e., one or more of the hydrogen atoms present in the groups are substituted by fluorine atoms. In that case, carboxyl groups and/or hydroxyl groups act as polar groups that are liberated following elimination of the acid-labile groups, if corresponding monomers have been copolymerized.
With the objects of the invention in view, there is also provided a process for structuring substrates. The first step is coating a substrate with a photoresist as described above to produce a photoresist film. The next step is exposing sectionally the photoresist film to light having a wavelength less than 200 nm. The next step is developing the exposed photoresist film is developed to form a structure from the developed photoresist. The next step is transferring the structure to the substrate.
In accordance with a further object of the invention, the process includes first applying a bottom resist to the substrate. A subsequent step is applying the photoresist to the bottom resist.
In accordance with a further object of the invention, during the exposing step, the photoresist film is exposed to light having a wavelength of 157 nm.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in chemically amplified photoresist and process for structuring substituents using transparency enhancement of resist copolymers for 157 nm photolithography through the use of fluorinated cinnamic acid derivatives, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments and examples.